Characterisation of semi-solid deformation behaviour of aluminium-copper alloys via combined x-ray microtomography and nite element modelling

The production of aluminium sheet is expensive and energy intensive despite the reduced environmental impact during use. Twin roll casting is a method of directly producing aluminium alloys in near net shape directly to sheet at a fraction of the energy costs of conventional DC casting / hot rolling. It also requires a fraction of the capital cost. Although sheet can be produced, defects (segregates, surface bleeds, buckling, etc.) can arise which limit the range of alloys which can be cast. This project aims to elucidate the complex mechanisms causing these defects through a combined experimental and computational study of semi-solid deformation in aluminium alloys. Columnar dendritic structures were generated for Al-Cu alloys through directional solidi cation experiments and quanti ed in three dimensions (3D) using x-ray microtomography (XMT). The -Al and the Cu-rich interdendritic liquid were segmented using image analysis. These 3D datasets were exported as meshes to be used in control volume and nite element codes. Firstly, the ow between the dendrites was simulated by solving the Stokes equation and permeability tensor was calculated as a function of the fraction solid. The size of representative volume element was estimated to be 4-6 times the characteristic length scale in the microstructure. Secondly, nite element simulations were performed on 3D columnar dendritic structures to estimate their mechanical properties and derive constitutive behaviour as a function of temperature, strain-rate and fraction solid. Temperature and strain-rate dependent compression tests were performed in the Gleeble on alloys with dendritic composition to determine the mechanical properties of the monolithic Al-dendrites. The fraction solid dependency term in the constitutive equation was determined as a purely geometric factor which could be easily replicated in other alloys systems. Lastly, hot tearing was directly observed in an Al- 12 wt.%Cu alloy by combining x-ray/synchrotron radiography with a new tensile/compression apparatus capable of measuring strain, load and quantifying the microstructure during controlled solidi cation of Al alloy specimen. Using this new apparatus, the deformation of primary dendrites and the concomitant ow of Cu-rich interdendritic uid was observed during isothermal and constant cooling rate conditions. Initially, strain was observed to be accommodated by liquid ow, but as the load is increased, void formation combined with liquid necking between grains was prevalent

Characterisation of semi-solid deformation behaviour of aluminium-copper alloys via combined X-ray microtomography and finite element modelling

The production of aluminium sheet is expensive and energy intensive despite the reduced environmental impact during use. Twin roll casting is a method of directly producing aluminium alloys in near net shape directly to sheet at a fraction of the energy costs of conventional DC casting/hot rolling. It also requires a fraction of the capital cost. Although sheet can be produced, defects (segregates, surface bleeds, buckling, etc.) can arise which limit the range of alloys which can be cast. This project aims to elucidate the complex mechanisms causing these defects through a combined experimental and computational study of semi-solid deformation in aluminium alloys. Columnar dendritic structures were generated for Al-Cu alloys through directional solidi cation experiments and quanti ed in three dimensions (3D) using x-ray microtomography (XMT). The -Al and the Cu-rich interdendritic liquid were segmented using image analysis. These 3D datasets were exported as meshes to be used in control volume and nite element codes. Firstly, the ow between the dendrites was simulated by solving the Stokes equation and permeability tensor was calculated as a function of the fraction solid. The size of representative volume element was estimated to be 4-6 times the characteristic length scale in the microstructure. Secondly, nite element simulations were performed on 3D columnar dendritic structures to estimate their mechanical properties and derive constitutive behaviour as a function of temperature, strain-rate and fraction solid. Temperature and strain-rate dependent compression tests were performed in the Gleeble on alloys with dendritic composition to determine the mechanical properties of the monolithic Al-dendrites. The fraction solid dependency term in the constitutive equation was determined as a purely geometric factor which could be easily replicated in other alloys systems. Lastly, hot tearing was directly observed in an Al- 12 wt.%Cu alloy by combining x-ray/synchrotron radiography with a new tensile/compression apparatus capable of measuring strain, load and quantifying the microstructure during controlled solidi cation of Al alloy specimen. Using this new apparatus, the deformation of primary dendrites and the concomitant ow of Cu-rich interdendritic uid was observed during isothermal and constant cooling rate conditions. Initially, strain was observed to be accommodated by liquid ow, but as the load is increased, void formation combined with liquid necking between grains was prevalent.EThOS - Electronic Theses Online ServiceALCOA and EPSRCGBUnited Kingdo

Microtomographic characterization of columnar Al–Cu dendrites for fluid flow and flow stress determination

During the twin roll casting of Al alloys, the interdendritic liquid may flow as the two solidification fronts are compressed together between the rolls. This can lead to defects such as centerline segregation. To understand the flow properties of the interdendritic liquid, samples of Al–12 wt.% Cu were solidified directionally in a Bridgman furnace and quenched to capture the growing columnar dendritic structures. The quenched samples were scanned using a laboratory X-ray microtomography (XMT) unit to obtain the 3D structure with a voxel resolution of 7.2 μm. Image analysis was used to separate the Al dendrite from the interdendritic Al–Al2Cu eutectic. Flow between the dendrites was simulated by solving the Stokes equation to calculate the permeability tensor as a function of the fraction solid. The results were compared to prior experimental measurements and calculations using synchrotron tomography observations of equiaxed structures. Elasto–plastic finite element (FE) simulations were performed on the dendritic structures to determine flow stress behavior as a function of fraction solid. It was found that the standard approximations for the reduction in flow stress in the semi-solid have a variation in excess of 100% from that calculated using the true structure. Therefore, it is critical to simulate the actual dendrite for effective flow stress determination

Quantifying the Effects of Grain Refiner Addition on the Solidification of Fe-Rich Intermetallics in Al–Si–Cu Alloys Using In Situ Synchrotron X-Ray Tomography

The presence of Fe-rich intermetallics, particularly β-Al 5 FeSi, in aluminium alloy cast components can often limit fatigue life. There is an on-going effort to control the formation of these detrimental phases through the additions of trace elements and grain refiners. However, the role of grain refinement on the formation of intermetallics is still unclear and conflicting results exist. To gain better understanding, in situ synchrotron X-ray tomographic microscopy experiments were performed on a commercial Al–Si–Cu alloy with grain refiner addition. Three-dimensional microstructure evolution and intermetallic precipitation were quantified. The influence of the β-intermetallics on the evolution of permeability during equiaxed dendritic solidification was also investigated numerically. The results illustrate that grain refinement affects α-Al grain structure as well as nucleation temperature of primary and intermetallic phases, but there is no evidence that it alters the precipitation sequence of intermetallics or their morphology. The simulation results reveal that intermetallics block interdendritic liquid flow and hence reduce permeability